This invention relates to a method for measuring the resistance component leakage current included in a leakage current to monitor the insulation of a distribution system, and relates to an instrument, apparatus, or system to which this measurement method is applied.
(A) A direct measurement method in which a zero-phase current transformer is provided to a distribution circuit or a grounding conductor of a transformer, (B) a measurement method in which a voltage is applied externally on a grounding conductor of a transformer or a distribution circuit, or (C) a method in which a leakage current is measured based on the output of a zero-phase current transformer and the voltage of a distribution circuit have been proposed as the method for measurement of the leakage current or insulation resistance of a conventional distribution system.
The exemplary disclosure that belongs to (A) includes Japanese Patent Laid-Open No. H2-129556, the exemplary disclosure that belongs to (B) includes Japanese Patent Laid-Open No. S63-238470, Japanese Patent Laid-Open No. H1-143971, Japanese Patent Laid-Open No. H2-83461, Japanese Patent Laid-Open No. H4-52565, Japanese Patent Laid-Open No. H6-258363, Japanese Patent Laid-Open No. H9-318684, and Japanese Patent Laid-Open No. H11-304855, and the exemplary disclosure that belongs to (C) includes Japanese Patent Laid-Open No. H3-179271, Japanese Patent Laid-Open No. H4-220573, Japanese Patent Laid-Open No. H6-339218, Japanese Patent Laid-Open No. H2001-225247, Japanese Patent Laid-Open No. H2001-21604, Japanese Patent Laid-Open No. H2001-215247, and Japanese Patent Laid-Open No. H10-78462.
These disclosures are summarized in FIG. 9.
FIG. 9 is a structural diagram showing a leakage current measurement system. In FIG. 9, 40 denotes a transformer, 41 denotes a breaker, 42 denotes a primary circuit of a distribution system, 43a, 43b, and 43c denote loads of electric components, 44 denotes a grounding conductor of the transformer 40, 45 denotes an apparatus for measurement of the leakage current based on the received output of a current transformer 46, 47a, 47b, and 47c denote electrostatic capacities generated on the distribution path, 48a, 48b, and 48c denote power switches of the loads 43a to 43c, and 49 denotes an electrostatic capacity of a noise filter provided to the load. 50 denotes a voltage application apparatus for applying a voltage on the ground conductor 44 of the transformer 40, and 51 denotes the insulation resistance of the load 43a or the insulation resistance of wiring of the primary circuit 42 for the purpose of convenience.
Iz denotes a leakage current of the primary circuit, Ic denotes a capacitative leakage current (reactive leakage component current) that flows in the electrostatic capacity, and Igr denotes a resistance component leakage current (active component leakage current) that flows in the insulation resistance component.
In FIG. 9, in the above-mentioned measurement method (A), the zero-phase current transformer is provided to the grounding conductor 44 of the transformer 40 to measure the leakage current. In the measurement method (B), a voltage of about 1 Hz/1 V is applied from the voltage application apparatus 50 so that a current does not flow to the electrostatic capacities 47a to 47c to eliminate the effect of the electrostatic capacity, and a signal generated from the zero-phase current transformer 46 is measured by use of the instrument 45. In the measurement method (C), the measurement is carried out based on the voltage applied from the primary circuit 42 of the distribution system and the output of the zero-phase current transformer 46.
FIG. 10 is a vector diagram showing the vectors of the leakage current Iz, the capacitative current that flows in the electrostatic capacity, and the resistance component leakage current that flows in the insulation resistance component. In FIG. 10, the phase angle between the phase voltage and the line voltage is 30 degrees in the case of the three-phase alternating voltage. The capacitative current Ic is 90 degrees different from the resistance component leakage current Igr, and the leakage current Iz is the composite current of the current Ic and the current Igr, namely vector sum. However, the capacitative current varies depending on the magnitude of the loading. For example, when all the loads 43a to 43c are loaded, the capacitative current increases as shown with Ic′. As the result, the leakage current Iz changes to Iz′. In other words, the currents Iz′ and Ic′ change depending on the variation of the load.
The above-mentioned method (A) is involved in a problem of incapable of measurement of the resistance component leakage current if the electrostatic capacity is large due to a noise filter because the reactive leakage component current is predominant.
The above-mentioned method (B) is also involved in a problem of complex system structure due to the requirement of external application of a voltage and the requirement of no effect on the loading apparatus.
One exemplary method of the above-mentioned method (C) is involved in a problem of unsuitability for a plurality of distribution circuits because an auxiliary impedance element is provided and the insulation resistance is determined.
Another exemplary method of the above-mentioned method (C), in which the phase angle is determined to calculate the resistance component leakage current and resistance value and to further detect the insulation deterioration phase, is involved in the difficulty in determination of the correct phase angle in the small current region because of the characteristic of the current transformer.
It is also necessary to measure the voltage level of the target measurement circuit, and a component for this purpose is required, resulting in problems of greater complexity and a higher cost. On the other hand, a zero-phase current transformer widely differs in phase characteristics from product to product, resulting in a problem of poorer accuracy and incapability for general use in combination with others.